Portable tools such as chain saws or power cutters are used in many different handling positions, even upside down. They are therefore usually crankcase scavenged and lubricant, e.g. oil is supplied to the crankcase. This lubrication system works in every handling position. These engines are usually of two-stroke type, but also four-stroke engines have been suggested.
U.S. Pat. Nos. 4,708,107, 5,347,967, 5,579,735 and 6,145,484 and DE 30 22 901 show crankcase scavenged oil in fuel four-stroke engines. A flow is arranged via the valve drive assembly housing into the combustion chamber via the intake valve. This system provides good lubrication but gives a strong heating of the intake air being in contact with very hot engine parts, e.g. valve stems. This reduces the power output. This applies also for a part flow in U.S. Pat. No. 6,401,701, see FIGS. 3 and 6 and DE 34 38 031. This reduces the power loss but it requires a complex structure of the intake system.
There are also designs that are not crankcase scavenged and lubricated. In these designs an oil sump or tank is filled with oil to a recommended oil level. Therefore there is no need to supply oil to the fuel. But instead they need an oil tank or oil sump of considerable size. This tank or sump usually reaches well below the level of the crankcase. Therefore the size and the weight of the engine is increased considerably. Examples are EP 1 134 365 and EP 1 136 665. They also use special extra parts for creating oil mist.
U.S. Pat. No. 6,152,098 shows a design with an oil sump of considerable size reaching down well below the crankcase. It is to be filled with oil to a prescribed level. Weight and size is somewhat reduced compared to the EP application-designs above, but is still a problem. The engine is crankcase scavenged using pure fuel, i.e. not mixed with oil.
U.S. Pat. No. 6,145,484 shows some versions (see FIGS. 6 and 7) that are not crankcase scavenged but have ducts leading from the intake duct both to the crankcase and to the valve drive assembly in a complex manner.
U.S. Pat. No. 6,199,532 shows an engine wherein a connecting channel is arranged between the top section of the valve drive housing and the intake duct. The flow connections from the crankcase to the intake channel are free from valves and control elements. This system is simple but vulnerable to pumping in the crankcase making fuel and lubricant flow back to the carburettor and air filter, a so called back spit.